Tool changers for changing a tool on a spindle of a machine tool are known from the prior art, see e.g. EP 577 850 A1, EP 1 652 617 A1 and EP 1 839 806 A1. Such tool changers comprise a tool drum having a plurality of tool holders disposed on the circumference of the tool drum for holding tools, and an axle carrier on which the tool drum of the tool changer is rotatably supported for aligning the tool holders with the spindle by rotating the tool drum. The alignment by rotating the tool drum and the driving of the rotation of the tool drum, respectively, for aligning the tool holders with the spindle is performed by the spindle of the machine tool so that no additional drive has to be provided.
Thus, the drum rotation of the tool drum and the positioning of the tool holders with the spindle required for the tool selection is performed by the main spindle drive, wherein a spindle gear attached to the spindle at a tool changing position of the tool changer relative to the spindle stock of the machine tool meshes with a gear of the tool changer and effects a rotation of the tool drum by rotating the spindle.
However, as the torque of the main spindle drive of a machine tool required for rotating the drum is usually in a high rotational speed range, the speed reduction of such gear drive will not suffice to drive the tool drum for rotatably aligning the tool holders. For this reason, a step-down gear, which is formed as a cam step-down gear, is additionally integrated in the tool drum in the prior art.
EP 1 652 617 A1, for example, shows a cam step-down gear comprising a driving plate having a ball driving groove, and a driven plate which also has a cam driving groove. Furthermore, a ball guide having radial ball guiding openings is provided between the driving plate and the driven plate.
Due to the different designs of the respective ball-driven grooves of the driving plate and the driven plate, a speed reduction is achieved by effecting a radial movement of the balls in the radial ball openings of the ball guide by rotating the driving plate, the radial movement of the balls in the radial ball openings then being converted into a rotary movement of the driven disk at a reduced speed. In this case, the driving plate is driven by the spindle of the machine tool via a gear transmission, and the driven plate is connected to the tool drum in a rotationally fixed manner such that the driving of the driving plate effects a rotational movement of the tool drum at a reduced speed. Such cam step-down gear is known, for example, from JP 60205058.
However, such tool changers having a cam step-down gear of the above-described type have various drawbacks. On the one hand, the structure of such cam step-down gear is cumbersome and space-consuming because the driving disk, the central ball guide and the driven disk must be provided with a large circumference. Furthermore, due to the transmission of a rotational movement into a radial movement of the balls into another rotational movement, a high friction effect is caused which must be overcome by the spindle drive and furthermore leads to increased wear. Moreover, for driving the tool drum by means of the spindle of the machine tool it is necessary to provide an exposed spindle gear in the work space of the spindle, that is, in the range of the tool holder of the spindle, via which the driving disk of the cam step-down gear is driven.